Protective metal coatings for molybdenum



r r fie qf l Q PROTECTIVE METAL COATINGS FOR? Gl n scram Jan-Lo an i'e mm ri'sgohi assignors,'jby inesnea'ssigrirnfe'nts, to thefUnited St tes' of America as represented by the Secretary' -'ofthfe Application January 7, 1957, Serial mama ij 'C' a S-i "(c 1'294i; 41); l

if p esent iflvenaah relatest'o jrnetal molybdenum andmore particularly to electroplated co denumflthroughthe,coatings,", p It;i we n knownthebmolybdenum, a well other I fractoiiy metals," is readily oxidize'd when' exposed, tofaii irigs thatf'will act, asa barrierto th'e'fditfusion ofjm'olylb oxygen bearing v-a'tmosphere' at, elevated temperatures;

molybdenum,- and. molybdenum base'falloys have high strength at elevated te'rnperatures',v it is desirable tolvjp'ro vi de satisfactory? oxidation resistance 7 for these f materials O thattheir use can be expanded, It hasbeen thefcuspracticeto electroplate molybdenum withvarious other metals, such as;n'ickel .and chromium, topreventthe oxidationofth'e molybdenum. I A protective electroplated metal at a hightemperature must not only 're'sist' oxidation from the environment in Which the coating will be placed but must, also prevent deterioration of the basis metal caused by the environment. The oxidation resistance of most commercially usable electrodeposited'metals is dependent upon the formation of any adherent and impervious oxide coating. In addition to oxidation-resistance,

operating condition'sr the electroplate and its scale must withstand impact arid abrasion, .stress corrosiona nd creep? If, the electroplate difi'usesrapidly with the basis metal, the coating Flif is shortened. Discontiriiiities"inpthe coating reduce-- its ability to protect t h e basis metal, and therefore the density and structure of electrodeposits are important.

Although nickel resists oxidation in air better than chromium under conditions of cyclic heating, chromium can be deposited on molybdenum with a better adherence than is possible by plating nickel and several other electrodeposits directly on molybdenum. However, at high temperatures, the molybdenum ditfuses through the chromium plating, causing shrinkage of the molybdenum, and, furthermore, molybdenum and/or oxygen diffusing through the plating and reacting with each other at high temperatures forms a liquid or gas that corrodes chro- Inium, nickel, and other metals faster than does air.

The present invention provides an improved method for coating molybdenum and molybdenum base alloys by first depositing chromium on molybdenum and then applying a deposit of gold over the layer of chromium. A protective layer of chromium can then be deposited over the layer of gold. At elevated temperatures, gold only slightly retards the diffusion of chromium through the gold and into the molybdenum, but diffusion of the molybdenum through the resulting gold-chromium diffusion alloy is eifectively retarded.

It is therefore a general object of the present invention to provide an improved coating for molybdenum whereby the rapid oxidation of the molybdenum at high temperatures is efiectively retarded.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing wherein:

f Figure 111s], partial Sama a 'view'sh wmg jiimmiumf an ld 'pla'tin ori'a molybdenumfspecimen;

at f -.?IPlyin' tri e, o pprQ 005 inch) 'of chromium 11 'on the molybdenum r t hat thejsu bls'equent gold plate 13 will be fde sit ,adhere'ntly, The basev metalis cleansed and pre ararerpnfin i any conventional manner and then with, chromium "B way 'of example, the molyb 'pbje'cts' u e, p d can bejhe'attreated in hydrof hours at 9801C; cooled, immersed ininitric'acid n V(3 HNQ 11-1 0) 'for' 10{second s', rinsed, electric 2 mat s, tisso'j mp/sqgrt. 5" 'i'na 180111 min ,100 gg/lich gm iea id and 10 g./l."sodiurn litihfomate msedQimmersed m 1s percent, by'weight,of hydrochlor c acid solution, rinsed, and plated with chro f at .185 Fiwithga'current density 013 990 sq; ft.for 1"to5mini1tes'." f 7 After, the molybdenum specimens have been' given 'a 'sltrikecoating of-chromium',' a' .0015 inch goldla'yer is dep s tedyw yf am e1 f p im s ra i k inersedfin 15 pereent, by weight, hydrochloricifacid solu; fiqi for se dn lat f F-,; and t n-P in a gold plating bath, having, the following forrrlulatiori and Formulation .-Operating conditiOnsa, Cathode current density 5 or 10 amp./ sq. ft. Anodes 24K gold.

Bath temperature F.

Agitation None.

Plating rate 0.0015 inch/hour at 10 amp./sq. ft.

After plating with gold, a heavier coating of chromium 14 (.003-005 inch) is then deposited over the layer of gold. By way of example, the heavier layer of chromium 14 can be plated by using the following formulation and operating conditions:

Formulation 250 g./l. chromic acid 2.5 g./l. sulfuric acid Operating conditions Cathode current density 750 amp./ sq. ft. Anodes Chemical lead sheet. Bath temperature F. Plating time Approximately 3-5 hours.

Referring now to Figure 2 of the drawing, there is shown a view, as might be observed under a microscope, of an electroplated molybdenum specimen that has been heated, wherein the heating caused a progressive growth of a molybdenum-chromium diffusion zone 15 under the gold. Microscopic thickness measurements of sections heated for 25, 57, and 100 hours showed that the alloy zone grows at nearly the same rate as the diffusion alloy grows between molybdenum and chromium, as shown in ..rae emw12 1959.

o a1 vi w illustr't ng a mic'roscopic the drawingithe fpre'sent inven-l Figure 3 of the drawing. For example, one molybdenum specimen that was first given a".0005 inch thick chromium strike, followed by a .0015 inch plating of gold, and then .004,.inch of chromium, was heated 91 hours at 980 C. in argon atmosphere, and the .diitusion .Zon in r the gold and chrominniliplated .rno'ly d 'n m semen;

ifany, gold because th interm tallie compound Ql 39 d and molybdenum-a e verymnchha der T ha dn ss of ey w-gold.layer'lfiabove th n molybdenumzone ,15 indicates that, upon he t ng. a si n icant proportion of chromium is alloy with the gain. ".Iih r hardnessof the gold-chromium alloy .6 5 K9999, I

whereas annealed gold has a hardness of le s .IhanFlO Knoop. 'Thehardness of therchrominmu abo e't e gold a y is QP, and, because th s value corresponds to the hardness-of annealed, electrolyticchrorniurnyit is outer layer .of chromium.- Thu t i vi en that the molybdenum base metal 12 adequate YQmt ted from oxidation at high temperatures by the layers of chromium and gold.

f During microscopic examination 9f e ctr plated moly p i n several gbPs inth gold 'alioy layer were observed. Whenever the gaps appeared, the white diffusion alloy below. the gold a loy was very narrow. These observations show that the gaprctarded the diffusion of chromium with molybdenum; "Because the growth of the white diffusion zone depended upon the diffusion of chromiumthrough gold, the white diffusion zone evidently contained.little,;if..any, gold.

. 4 v more resistant than nickel to oxidation in air, nickel is actually more resistant under conditions of cyclic heating, the reason for this apparently being due to the fact that the nickel forms a tightly adherent nickel oxide surface coating. On the other hand, the chromium oxide scale formed on the chromiumsurfage is not as adherent as the nickel oxide scale fonrned'on the nickel surface.

Obvious y .niany modifications and va iations; of the present invention are possible in the light of the above eachi gs.- .It sho ld the e o e b understood h t it is in d th ll mat rcont ned n the n nvedenerinti and a ng sha l be int rp eted as llust nt ve'an n in a limiting sense. i

What is claimed is:

1. The methodof protecting molybdenum articles from oxidation at high temperatures comprising, first electroplating said articles with a strike coating of chromium about .0005 nch t i next electropla ng a l er of gold about .9915 in h thic on said S ri e Jenn rch omium and fina y e ec oplatin l yer of 9L 243 1 3! from .003 to .005 inch thickon'saidlayer of gold, zrThe rnethod o protecting arti les o mol denu an molybdenum ba a loys .f e n oxidat onn h gh ternevident that no molybdenum or gold d fl nses into the pe ures compris g, l c rop at n a st ik 9. 32% of abo t 1 0 nch thicken sa d rtielenlnert e ec oplatin a a non o sold ab ut 9.01. n hith li t ere n, hen e ect platin a subst nt l y thieker atn f chromi m o 9. 3 to v0 inch hinknns is 5 4 seating. and hen electropla in tberen er an ou bea ing o a met re istan t x a ion u der W91}? eat n Ibe m thod o p o tin a ti les i .n b ybdenuni and ybdennm base a loy 'a s t fo th-1 sl in .2

whe einsaid outside c at o eta i ni kel- After molybdenum .or molybdenum base alloys are coated with a diffusion barrier as .describedabove it may be desirable to apply an additional oxidation resistant metal over the outer coating of chromium 14, In particular, under conditions of cyclic heating, ,theielectroplating of a nickel coating over the chromium coating 14 is highly desirable. Although chromium is 'ordinarily considered References Cited the file of this patent N TE STAT S A ENTS I Antisell Apr. 12, 1938 

1. THE METHOD OF PROTECTING MOLYBDENUM ARTICLES FROM OXIDATION AT HIGH TEMPERATURES COMPRISING, FIRST ELECTROPLATING SAID ARTICLES WITH A STRIKE COATING OF CHROMIUM ABOUT .0005 INCH THICK, NEXT ELECTROPLATING A LAYER OF GOLD ABOUT .0015 INCH THICK ON SAID STRIKE COATING OF CHROMIUM, AND FINALLY ELECTROPLATING A LAYER OF CHROMIUM FROM .003 TO .005 INCH THICK ON SAID LAYER OF GOLD. 